In U.S. Pat. No. 4,799,625, assigned to the same assignee as the assignee of this application, (and fully incorporated herein by this reference) there is disclosed a method and apparatus in which the adjustment between a rotating cutterhead and a shearbar in a forage harvester is achieved automatically under the control of a microprocessor. According to this prior art patent, insofar as relevant for this application, the adjustment process is controlled in part by sensors and procedures which assure that no adjustment process is allowed to proceed unless the sensors have determined that the cutterhead is rotating at or above a predetermined minimum. As pointed out in the patent, the adjustment mechanism may damage either itself or the cutterhead if the cutterhead is not rotating.
In the prior art patent, the rotation of the cutterhead is determined by a tachometer which generates a pulse-train to indicate rotation of the cutterhead. Electrical signals which represent such pulse-trains, even at relatively low repetition rates, contain significant high-frequency components and these high-frequency signal components, even at relatively low power levels, will tend to "leak" from their assigned signal paths into adjacent signal paths and components. Such leakage may effect the operation of adjacent components and cause erratic functioning of certain circuit elements.
In the case of agricultural equipment, such as forage harvesters, which operate under difficult environmental conditions of vibration, temperature, and dirt, the problems are compounded.
Accordingly, it is a primary object of this invention to improve the method and apparatus for adjusting a shearbar relative to a cutterhead which is not based on pulse electrical signals to determine rotation of the cutterhead, to thereby eliminate malfunctioning due to signal leakage.
In the forage harvester described in the aforesaid patent, the cutterhead is driven (through a selectively engagable clutch) from the prime mover of the forage harvester. To assure that the cutterhead is rotating, it is therefore a necessary element that the prime mover be rotating. Rotation of the prime mover can be inferred from a number of sensors, some of which generate pulse trains (e.g. an alternator) and some of which generate a more or less steady state signal without appreciable transients (e.g. oil pressure sensors).
It is another object of this invention to infer the rotation of the prime mover of a forage harvester from a more or less steady state signal without significant transients.